Huntington's disease belongs to a family of eight inherited, untreatable neurodegenerative diseases. Each is caused by an abnormal polyglutamine expansion in a different protein, In all eight, disease occurs when the polyglutamine stretch exceeds a certain length and symptom onset is inversely related to its length. Abnormal deposits of protein called inclusion bodies characterize many of these diseases. Whether Inclusion bodies are pathogenic, an epiphenomenon, or a beneficial defense response is controversial. Abnormal polyglutamine expansions probably cause degeneration by conferring a toxic gain of function to proteins. Polyglutamine expansions may adopt a conformation that is different depending on whether huntingtin is in Inclusion bodies or not. We hypothesize that abnormal polyglutamine expansion alters the conformation of soluble mutant huntingtin, enabling it to interact with cellular targets and produce neurodegeneration, independent of inclusion body formation. Certain heat shock proteins can protect cells against polyglutamine and regulate inclusion body formation. It is unknown whether they act mainly on inclusion bodies, oligomers, or malfolded monomers. It is unknown whether huntingtin needs to oligornerize to adopt a toxic conformation and whether the length or the composition of the polyglutamine stretch is critical to conformation or aggregation. We have used primary neurons to develop a model of Huntington's disease that recapitulates polyglutamine-dependent and neuron-specific death and inclusion body formation. We have also developed monoclonal antibodies that bind a conformation of huntingtin that correlates closely with Huntington's disease symptoms and which distinguishes huntingtin in inclusion bodies from more soluble forms. Finally, we have built a robotic microscope so we can simultaneously measure inclusion body formation within living neurons and then track their individual fates. We propose to use these tools to accomplish the following specific aims: Aim 1. To determine whether the availability of a disease-associated conformation is a better predictor of neurodegeneration than inclusion body formation. Aim 2. To determine whether heat shock proteins regulate huntingtin conformation and to relate the effects of heat shock proteins on neuronal survival to conformation and to inclusion body formation. Aim 3. To examine how the oligomeric state of mutant huntingtin and the length and composition of the polyglutamine stretch determine its ability to form a disease-associated conformation and to aggregate.